1. Field of the Invention
This invention relates to etching methods for etching a material to be shaped (such as a crystal substrate) into a predetermined pattern, and also relates to etch products (such as a crystal wafer) shaped by these methods. In particular, this invention is concerned with measures for simplifying control operations in an etching step.
2. Description of the Related Art
Among conventional piezoelectric devices, a tuning-fork crystal transducer is easy to miniaturize. As disclosed in Japanese Patent Laid-open Publication No. H10-294631, this type of transducer is equipped with a tuning-fork crystal oscillator in which a crystal wafer is processed in the shape of a tuning fork by etching and provided with prescribed surface electrodes by photolithography.
Japanese Patent Laid-open Publication No. 2002-76806 (herein after mentioned as “Prior art document 1”) discloses a tuning-fork crystal oscillator whose legs have grooves formed in the central parts of their front and back surfaces (major surfaces). The grooves formed in the front and back surfaces of the legs are effective because, even in a miniaturized oscillator, they can reduce loss of oscillations in the legs and can ensure a low CI (crystal impedance). The tuning-fork crystal transducer of this type is particularly suitable for being mounted on a precision instrument such as a watch.
Regarding this tuning-fork crystal wafer whose legs have grooves formed in their front and back surfaces, its shaping process is described according to the method disclosed in Prior art document 1.
To start with, a crystal substrate (Z-plate quartz crystal) 100 is processed into a plate as shown in FIG. 23(a). Then, using a sputtering apparatus (not shown), metal layers 101, 101 of Cr (chromium) and Au (gold) are deposited over the front and back surfaces of this crystal substrate 100 (see FIG. 23(b)). Photoresist layers 102, 102 are formed on these metal layers 101, 101 (see FIG. 23(c)).
Next, as outline patterning, the photoresist layers 102 are partially removed in such a manner that the photoresist layers 102, 102 remain on oscillator-shape areas 103 which correspond to the pattern of an intended tuning-fork crystal wafer (the shape of a tuning fork) and on frames 104, 104 which define the outer border of the crystal substrate 100. FIG. 23(d) and FIG. 24(a) are a sectional view and a perspective view showing this stage, respectively. As illustrated in FIG. 24(a), the photoresist layers 102, 102 constitute reliefs of a tuning-fork crystal wafer in a prescribed pattern.
Moving next to FIG. 23(e), the metal layers 101 are etched away in a Au etchant and a Cr etchant, from the parts where the photoresist layers 102 were removed in FIG. 23(d). Thereby, as shown in FIG. 24(b), the crystal substrate 100 is exposed at the parts where the metal layers 101 were etched away.
Turning then to FIG. 23(f), the photoresist layers 102 which were retained in FIG. 23(e) are removed completely. After that, the crystal substrate 100 is entirely covered with photoresist layers 105 as shown in FIG. 23(g).
Further, the photoresist layers 105 are partially removed as shown in FIG. 23(h). To be specific, the photoresist layers 105 are removed except on the oscillator-shape areas 103 and the frames 104. In addition, as groove patterning, the photoresist layers 105 are removed from the parts to be grooves 106 (see FIG. 23(k)).
Thereafter, as shown in FIG. 23(i), outline etching is performed with use of a crystal etchant, whereby only the oscillator-shape areas 103 and the frames 104 are retained.
Further, as shown in FIG. 23(j), the metal layers 101 are etched away in the Au etchant and the Cr etchant, from the parts to be the grooves 106 in the legs of the tuning-fork crystal wafer.
The resulting crystal substrate 100 is immersed in the crystal etchant for a preset period of time, until the crystal substrate 100 is etched to a predetermined depth. Thereby, the legs acquire grooves 106, 106, . . . in their front and back surfaces, and a generally H-shape cross-section. Finally, as shown in FIG. 23(k), the remaining photoresist layers 105 and the metal layers 101 are removed to produce a tuning-fork crystal wafer whose legs have a generally H-shape cross-section.
In the thus obtained tuning-fork crystal wafer, certain electrodes are provided on each surface of oscillation areas to make a tuning-fork crystal oscillator. In turn, this tuning-fork crystal oscillator is mounted inside a package to give a crystal transducer.
In connection with the tuning-fork crystal wafer whose legs have grooves 106, 106, . . . in their front and back surfaces, the grooves 106 should be processed with an extremely high precision for the following reasons.
Firstly, in the tuning-fork crystal transducer having the grooves 106, the oscillation frequency tends to be more variable than in the one without grooves 106. An effective means to reduce this tendency is to process the grooves 106 with a high precision.
Secondly, the tuning-fork crystal transducer having the grooves 106 ensures a low crystal impedence (CI). In order to ensure a low CI effectively, the grooves 106 need to be processed with a high precision.
Additionally, although the above description is focused on the outline processing and groove processing for a tuning-fork crystal wafer, high-precision processing is similarly preferable for other crystal wafers (e.g. an AT-cut crystal wafer).
Nevertheless, the method disclosed in Prior art document 1 has some problems as below.
In the etching step of forming grooves 106, 106, . . . in the front and back surfaces of the legs (the step of immersing the item shown in FIG. 23(j) into the crystal etchant), the crystal substrate 100 is etched deeper as the time passes.
Hence, in order to process the grooves 106 to an optimum depth as designed, the time for immersing the crystal substrate 100 in the crystal etchant (the etch time) has to be under strict management. Namely, if the etch time is too short, the grooves 106 are not deep enough. On the other hand, if the etch time is too long, the grooves 106 become too deep. In some cases, the grooves 106, 106 in the front and back surfaces may even penetrate into each other to form through-holes in the legs. Owing to this requirement, the method disclosed in Prior art document 1 involves complicated management of the etch time, which deteriorates workability.
Where better productivity is particularly intended, a crystal etchant which effects a great amount of etching per unit time may be employed to finish formation of the grooves 106, 106, . . . in a short time. In this case, however, slightest deviation from the optimum etch time causes the depth of the grooves 106 to deviate significantly from the design depth, ending with a deficient crystal wafer. Thus, for the method disclosed in Prior art document 1, it is difficult to enhance productivity and yield of the crystal wafer at the same time.
Generally speaking, the CI can be decreased in a tuning-fork crystal wafer of a relatively large size. In this type of crystal wafer, grooves are etched in the front and back surfaces of its legs, with the bottom of each groove having a relatively large area. In this connection, it should be kept in mind that a crystal wafer generally contains pits which are generated at a plurality of locations during the crystal growth process. These pits do not dissolve in a crystal etchant.
Hence, in the above-mentioned structure where the bottoms of the grooves have large areas, it is highly probable that many pits are exposed at the bottoms in the course of etching. If many pits are exposed at the bottoms of the grooves, the mass of the crystal wafer exceeds a target mass by the total volume of the exposed parts of the pits. Under such circumstances, the oscillation frequency of a crystal transducer cannot be tuned to a target frequency. Since the locations and the number of pits are unpredictable, it is difficult to reduce the number of exposed pits by simply adjusting the amount of etching (the etch time). Thus, according to the method disclosed in Prior art document 1, a crystal transducer cannot acquire a desired oscillation frequency at a high probability.
This invention is made in the light of these matters, and aims to provide an etching method for processing a material to be shaped (such as a crystal substrate). This etching method intends to dispense with the etch time management so as to simplify control operations in an etching step. Besides, when grooves are etched in a crystal substrate (a material to be shaped), this etching method intends to reduce exposure of pits at the bottoms of these grooves.